1. Field of the Invention
The present invention relates to an optical module for wavelength-division multiplex optical transmission and a method for manufacturing the same.
2. Description of the Related Art
In recent years, there have been made research and development of application of optical transmission systems to subscribers systems in its enforcement stage. What is indispensable in enforcement of bidirectional optical transmission for subscribers systems using wavelength-division multiplex is an optical module having functions of multiplexer/demultiplexer for combining optical signals of different wavelengths (multiplexing) and separating signals combined by multiplex (demultiplexing). Establishment of a mass producing technology of such optical modules is the attainment of one of the key technologies for enforcement of the optical transmission system for subscribers systems.
With reference to FIG. 1, structure of a prior art optical module for wavelength-division multiplex optical transmission will be described below. On a substrate 2 for common use for electronic circuits and optical circuits, there is provided an optical waveguide substrate 4. On the optical waveguide substrate 4, there are formed a plurality of optical waveguides 5, 5a, 5b, and 7. The optical waveguide 5 is divided into the optical waveguides 5a and 5b at a Y-branch portion 8. The optical waveguide 5 and optical waveguide 7 are disposed contiguously to each other at a directional optical coupler portion 6 having a demultiplexing function. The optical waveguide 5 has an input port 10 at its left end, while the optical waveguides 7, 5a, and 5b have a first, a second, and a third port 12, 14, and 16 at their right ends, respectively.
An optical signal, which is a wavelength-division multiplexed signal of an optical signal with a wavelength of 1.3 .mu.m and an optical signal with a wavelength of 1.55 .mu.m, is transmitted over an optical fiber 18. When the multiplexed optical signal is input to the input port 10, it is divided at the directional coupler portion 6 into an optical signal with the wavelength of 1.55 .mu.m and an optical signal with the wavelength of 1.3 .mu.m, of which the optical signal with the wavelength 1.55 .mu.m is propagated through the optical waveguide 7 and output from the first port 12. The optical signal with the wavelength 1.3 .mu.m is divided into two equal divisions at the Y-branch portion 8, which are propagated through the optical waveguides 5a and 5b and output from the second and third ports 14 and 16, respectively. Therefore, by disposing photodetectors opposite to the ports 12, 14, and 16, the wavelength-division multiplexed optical signal can be received.
In this prior art example, to make bidirectional optical transmission possible, the first port 12 is connected to a photodetector 20 capable of receiving the optical signal with the wavelength 1.55 .mu.m, the second port 14 is connected to a laser diode 24 outputting an optical signal with the wavelength 1.3 .mu.m, and the third port 16 is connected to a photodetector 22 capable of receiving the optical signal with the wavelength 1.3 .mu.m. Reference numeral 26 is an electronic circuit unit including a driver circuit of the laser diode, amplifiers for the photodetectors, etc.
In the conventional optical waveguide type optical module for wavelength-division multiplex optical transmission shown in FIG. 1, there is a demerit that it is difficult to obtain desired demultiplexing characteristics (wavelength dependence of the branching ratio) in a stabilized manner. Further, since it takes a long time in connecting an optical waveguide with an optical fiber, the module is not suited to mass production. Besides, there is such a demerit that the connection loss between the optical waveguide and optical fiber is great.